Peroxide dithiophosphates

The mechanisms of inhibition by peroxide decomposers, metal deactivators, and ultraviolet absorbers are fairly well understood in general terms, although many details of the individual reactions remain to be elucidated. Classifying a preventive antioxidant into one of the three categories above will only rarely describe its entire function. The dual behavior of dialkyl dithiophosphates in the liquid phase has been mentioned. Many other phosphorus- and sulfur-containing antioxidants commonly classified as peroxide decomposers can also act as chain breakers. Similarly, the structure of many metal deactivators and ultraviolet absorbers indicates that they must also have some chain-breaking activity. 

The kinetics of the zinc diisopropyl dithiophosphate-in-hibited oxidation of cumene at 60°C. and Tetralin at 70°C. have been investigated. The results cannot be accounted for solely in terms of chain-breaking inhibition by a simple electrow-transfer mechanism. No complete explanation of the Tetralin kinetics has been found, but the cumene kinetics can be explained in terms of additional reactions involving radical-initiated oxidation of the zinc salt and a chain-transfer step. Proposed mechanisms by which zinc dialkyl dithiophosphates act as peroxide-decomposing antioxidants are discussed. 

Although zinc dialkyl dithiophosphates, [(RO)2PS2]2Zn, have been used as antioxidants for many years, the detailed mechanism of their action is still not known. However, it is certain that they are efficient peroxide decomposers. The effect of a number of organic sulfur compounds, including a zinc dithiophosphate, on the rate of decomposition of cumene hydroperoxide in white mineral oil at 150°C. was investigated by Kennerly and Patterson (13). Each compound accelerated the hydroperoxide decomposition, the zinc salt being far superior in its activity to the others. Further, in each case the principal decomposition product... 

The inhibition of hydrocarbon autoxidation by zinc dialkyl dithiophosphates was first studied by Kennerly and Patterson (13) and later by Larson (14). In both cases the induction period preceding oxidation of a mineral oil at 155 °C. increased appreciably by adding a zinc dialkyl dithiophosphate. In particular, Larson (14) observed that zinc salts containing secondary alkyl groups were more efficient antioxidants than those containing primary groups. In these papers the inhibition mechanism was discussed only in terms of peroxide decomposition. 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

We have carried out a limited study of the effect of metal dialkyl dithiophosphates on a hydroperoxide-autocatalyzed oxidation system. Table III summarizes induction periods for the oxidation of squalane at 140 °C. These results do not unambiguously reflect the peroxide-decomposing property of each dithiophosphate radical capture also occurs. 

No readily acceptable mechanism has been advanced in reasonable detail to account for the decomposition of hydroperoxides by metal dialkyl dithiophosphates. Our limited results on the antioxidant efficiency of these compounds indicate that the metal plays an important role in the mechanism. So far it seems, at least for the catalytic decpmposition of cumene hydroperoxide on which practically all the work has been done, that the mechanism involves electrophilic attack and rearrangement as shown in Scheme 4. This requires, as commonly proposed, that the dithiophosphate is first converted to an active form. It does seem possible, on the other hand, that the original dithiophosphate could catalyze peroxide decomposition since nucleophilic attack could, in principle, lead to the same chain-carrying intermediate as in Scheme 4 thus,... 

The salts of alkyl xanthates, iV.AT -di-substituted dithiocarbamates and dialkyl dithiophosphates [43] are effective peroxide decomposers. Since no active hydrogen is present in these compounds, an electron-transfer mechanism was suggested. The peroxide radical is capable of abstracting an electron from the electron-rich sulfur atom and is converted into a peroxy anion as illustrated below for zinc dialkyl dithiocarbamate [44]... 

Both chain-terminating oxidation inhibitors, e.g. hindered phenols and amines, and peroxide-destroying inhibitors, e.g. dithiophosphate and dithiocarbamates, can be included in marine formulations. Mixtures of phenols and amines are often used for synergy but they must have good high-temperature performance. The sulphur-containing oxidation inhibitors also have extremely useful anti-wear properties. Oxidation inhibitors can be used advantageously in some base oils refined from low sulphur crudes and in synthetic basestocks. They compensate for the lack of natural antioxidant species. 

Some of the most powerful uv stabilisers belong to the class of peroxide decomposing preventive antioxidants and it has been suggested that the mechanism of this type of uv stabiliser is not distinguishable from their behaviour as thermal antioxidants although all peroxide decomposers do not behave as uv stabilisers (11). Of paramount importance in the peroxide decomposer -uv stabiliser class are the metal dithiocarbamates (III) (9,22,23,24) the dithiophosphates (IV) (11,24) and the... 

N-Cyclohexyl-2-benzothiazolesulfenamide 1,3-Dibutylthiourea Dicyclohexylbenzothiazyl-2-sulfenamide Dimethyl diphenyl thiuram disulfide 2,6-Dimethylmorpholine Diphenylamine 4,4 -Dithiodimorpholine 2-Ethylbutyraldehyde Ethyl morpholine Formaldehyde aniline 2,6-Lutidine N-Nitrosodimethylamine Paraldehyde p-Quinone dioxime Selenium Selenium dimethyidithiocarbamate Sodium diethyidithiocarbamate Sodium dimethyidithiocarbamate Sodium nitrite N,N,N, N -Tetrabenzylthiuram disulfide Tetraisobutylthiuram disulfide Tetramethylthiuram disulfide Thiocarbamyl sulfenamide Thiophenol Thiourea Triethanolamine Triethylamine Triethylenetetramine n-Valeraldehyde Zinc dialkyl dithiophosphate Zinc diamyidithiocarbamate Zinc dibenzyl dithiocarbamate Zinc diisobutyidithiocarbamate Zinc isopropyl xanthate Zinc 2-mercaptobenzothiazole Zinc oxide Zinc-N-pentamethylene dithiocarbamate Zinc peroxide accelerator, rubber articles for repeated use food-contact... 

Metal complexes of dithioic acids, eg, dithiocarbamates, dithiophosphates, xanthates (MDRC, MDRP, MRX, respectively) (see AOs 21-24, Table 3), which are highly effective catalytic peroxide decomposers (PD-C) and excellent melt stabilizers, are generally effective photostabilizers (101,105,136,137,139,149). Their... 

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